You ask a whole bundle of questions at once -- more than can be tackled in my response. Let's focus on two interrelated strands of your question: the badness of death and the wrongness of killing.
Suppose we could use chemical measures to make people (as you say) completely happy. You say such people would not have "instincts" and would not fear death. Both claims strike me as very implausible. A happy life seems rather to require instincts: instincts to pursue and engage in various activities that make us happy. Furthermore, I see no basis for thinking that a 'completely happy' person would not fear death. Exactly why people fear death is hard to pin down. But one plausible explanation is that they fear because death entails losing out on the further goods of life. But a completely happy person would have more reason to fear death, since (after all) they would stand to lose more than less happy, or unhappy, people would.
So I find the initial thought experiment to be, well, perhaps not impossible --...